The present invention relates to mesomorphic structures. More particularly, the present invention relates to mesophases based on polymerizable substituted carboxylic acids, for example substituted acrylic and methacrylic acids and their derivatives.
Liquid crystals, rather than being true liquids or true solids, occupy a state of matter somewhere between these two phases. True liquids are isotropic in that their molecules lack the ordered arrangement necessary to be a solid. In solids, the position of molecules is fixed in an ordered crystalline lattice or array. Like true liquids, liquid crystals exist in a liquid state. However, unlike true liquids but like solids, liquid crystals are characterized by the fact that their molecules are aligned in an ordered crystalline array. For this reason, liquid crystals are characterized as being anisotropic. The ordered regions in the liquids are referred to as mesophases while the individual molecular combinations that result in the mesophases are referred to as mesogens.
The type of molecules or mesogens that show a propensity for forming mesophases are typically characterized by an elongated or disk-like structure. Polarizability of the mesogen impacts the propensity for two molecules to be attracted to each other and thus assume an ordered state. One class of mesogen will therefore typically have a polar terminal group. A non-polar, often an alkyl or substituted alkyl, tail is typically provided at the other end of the mesogen. Thus, this class of mesogens will typically have a polar, hydrophilic group disposed at one end and a non-polar, hydrophobic tail disposed at the other end. However, mesogens are known which possess a central non-polar core and two polar ends or indeed vice versa.
Mesophases may form upon heating of the pure compound in which case they are referred to as being thermotropic. Alternatively, mesophases may form when the pure compound is placed in the presence of certain solvents in which case they are referred to as being lyotropic. The mesophases may be further classified according to the structure or orientation the individual molecules assume in the mesophase. In a smectic orientation, the mesogens are arranged in layers with respect to their centers of gravity. Moreover, the long axis of the mesogens are arranged in one preferred direction of alignment. In smectic systems, the lateral interactions between the mesogens are favored over the interactions between the layers. Thus, the smectic arrangement results in a characteristic fluidity evidenced by the slippage of one layer over another without the destruction of the order within each layer. In nematic systems, the long axes of the mesogens remain in a single preferred direction of alignment. However, the mesophase is no longer characterized by distinct layers of mesogens. In cholesteric systems, the mesophase is characterized by a helical structure. The helical structure is the result of a chiral center in the mesogen that causes the long axes of the mesogens to be rotated for successive layers.
The benefits of such an ordered system in a polymer matrix can be substantial. By polymerizing the liquid crystal system the ordered arrangement is frozen into place. Upon extrusion or processing, the rigid polymeric mesophases become aligned in the direction of flow, reducing frictional drag and viscosity. Upon cooling, the ordered arrangement is also maintained. The resulting polymer can exhibit greatly improved mechanical properties, including tensile strength and impact strength. The mechanical properties can also be tailored to confer anisotropic properties on the material. For example, the polymeric material could be prepared to resist shear in one direction, but to easily shear in the plane defined by the other two directions.
Polymerizable carboxylic acids, in particular acrylic and methacrylic acid, would upon initial observation appear ill-suited for the formation of mesophases. While exhibiting a polarizable acid group, these acids are not exemplematic of the elongated, disk-like molecules typically thought of as mesogens. ##STR1##
Typical derivatives of these carboxylic acid monomers, would also appear to be ill-suited to the formation of mesophases. These derivatives would include polymerizable salts of the acids such as sodium methacrylate; methyl and other alkyl derivatives of the acids such as poly(methyl acrylate) and poly(methyl methacrylate); and polymethacrylonitrile. ##STR2##
Some of these polymeric derivatives, namely the salts, the nitrile, and the shorter chain alkyl species, lack the elongated structure necessary for mesomorphic characteristics. Moreover, longer chain alkyl or similar derivatives, while having an elongated structure, are no longer characterized by one hydrophilic end and a second hydrophobic end. Thus, longer chain alkyl derivatives--esters--such as n-octyl methacrylate normally do not form single-phase lyotropic mesophases over a broad useful range of compositions.
Poly(acrylic acid) and poly(methacrylic acid) also do not exhibit the typical properties of liquid crystalline polymers. They are readily soluble in water and dilute bases. Their sensitivity to water renders the polymers impractical for most plastic applications. Likewise, rather than exhibiting excellent mechanical properties, poly(acrylic acid) and poly(methacrylic acid) are relatively brittle when in the dry solid state. Rather than melting and becoming thermoplastic upon heating, they crosslink, char, and degrade. Finally, in solution, they exhibit unusually high viscosities and for this reason are commonly used as thickening agents. They by no means possess the qualities of a lubricant.
Despite these problems, a polymeric derivative of these unsaturated carboxylic acids that exhibited mesomorphic properties would be extremely beneficial in certain applications.